Kit Hill is a magnificent granite plug hill in east Cornwall, north of Saltash and close to the Tamar Valley. It’s a ‘Marilyn’ [ see below ] that rises steeply up out of the rolling east Cornish countryside. And like so many of the Cornish granites it too played an important role in building London. Indeed it ” … was used [in] Millwall and Tilbury docks, Lambeth and Putney bridges in London .. London Bridge … Blackfriars, Battersea, Chelsea, and Waterloo bridges; Thames embankment, … and … Battersea Park Wall ” and ” The whole of the stone, some 70,000 cubic feet, for the bridge over the Thames at Battersea [ built 1885-1890 ] was supplied from these quarries. ”  For a small quarrying site this appears to have been a significant role!
Ruth Siddlall states “The Kit Hill granite deserves a mention here as it is a much ignored intrusion. It is a small stock, almost a kilometre in diameter which occurs near Callington, a few kilometres to the east of the Bodmin Granite, not far from the Devon Border. Bristow (2013) describes the appearance of the granite as ‘nothing exceptional’; it is a biotite granite, very similar to those quarried on Bodmin Moor and in the Carnmenellis Pluton. However it was extensively quarried as widely spaced joints enabled the extraction of large blocks. Kit Hill granite was much used as an engineering stone and was used for docks, bridges and lighthouses in the British Isles and even exported to Singapore.”  See also 
Before quarrying started Kit Hill was for exploited for hundreds of years for it’s ‘moorstone’, i.e large rocks lying above ground, and especially the classic Cornish/Devonian granite ‘tor’ or cheeses. Spalding states “There are 4,839 stone splitting pits recorded on Kit Hill” 
The big quarry to the north of Kit Hill my pictures are from opened in the the late 19thC, closing afaics in 1955 See Stanier’s awesome work here p. 455  and note he has many books on the subject available from all good booksellers!
The quarry used a 500m long incline tramway to send stone due north down the hill to the East Cornwall Mineral Line, opened 1872, to be loaded onto boats at Calstock Quay. The mineral line was linked to the mainline to Plymouth in 1908 and then stone would have presumably have bypassed Calstock. The line from Plymouth still exists as a passenger line to Gunnislake and looks a fantastic way to get to Kit Hill! [ see below ] 
Kit Hill also has cut but unused stone like Swelltor but I can’t find any references to there possible destination online, not even in Stanier who does mentions a coping stone at Kit Hill in Chapter 7 p. 256 but I think he is referring to another stone. 
Kit Hill is also famed for it’s mining history and is surmounted by an enormous 1858 chimney, part of the steam powered pumps used to drain the deep mines which were producing ores of “tin, copper, arsenic and wolfram ” 
Quarry or not Kit Hill, a country park since 1982, is definitely worth visiting for it’s stunning 360 degree views, north to Bodmin, east into Devon to Dartmoor and south to Plymouth and the sea down the Tamar Valley. And as I noted at the start it’s actually 1 of only 5 Marilyns in Cornwall. Marilyns are defined as ” … peaks with a prominence of 150 metres (492 ft) or more, regardless of height or any other merit ”   So In some ways it feels very different from the Cornish/Devon moors. The word Kit by the way is a reference to the birds of prey that can be seen on the hill. 
See details of the country park here – https://www.cornwall.gov.uk/media/znlphyc4/kit-hill-leaflet-09-print.pdf
Kit Hill is also of deep importance to Cornish history as the site of the defeat of a Cornish army by the Saxons in 9thC and is nowadays a site for one of the major Tansys Golowan or Cornish Midsummer Bonfires! 
Getting there Public Transport – The 12 from Plymouth/Saltash goes to Callington 7 days a week and Monday to Saturday the 12A goes to the bottom of the road up the hill
Better would be to take the train up the Tamar Valley to Gunnislake, again from Plymouth/Saltash.
Cycling – very doable from Plymouth/Saltash or more simply from Gunnislake
Walking – there are no direct footpaths to the country park
Car – surprisingly it is possible to drive to the top of Kit Hill. There are a number of small car parks on the way up.
 https://www.cornwall.gov.uk/media/ybdlziqy/evidence-report-the-building-stones-of-cornwall-the-identification-of-heritage-quarries.pdf and https://www.geolsoc.org.uk/~/media/Files/GSL/shared/Sup_pubs/2013/18675_file.rtf  https://www.ucl.ac.uk/~ucfbrxs/Homepage/walks/Embankment.pdf  https://ougs.org/southwest/event-reports/154/pre-agm-trip-to-kit-hill/  http://spaldingassociates.co.uk/wp-content/uploads/2014/02/pits_quarries.pdf
 Stanier, Peter (1985) The granite industry of South-West England, 1800-1980: A study in historical geography https://eprints.soton.ac.uk/413336/1/85075041.pdf  https://www.cornwall-calling.co.uk/mines/callington/kit-hill-consols.htm  https://en.wikipedia.org/wiki/List_of_Marilyns_in_the_British_Isles  https://www.theguardian.com/travel/2020/feb/17/walking-the-marilyns-hills-of-the-south-downs  https://en.wikipedia.org/wiki/Kit_Hill  https://thisissallybell.wordpress.com/2016/06/22/tansys-golowan-the-midsummer-eve-bonfire-2/